The manufacture of paper involves many steps where the paper web is run between a nip formed between two rolls or between a roll and a shoe of an extended nip press. The purpose of a nip between rolls, or between a roll and a shoe, is to exert pressure on the paper web. Because modern paper manufacturing machines are up to four-hundred inches or more wide, loading the ends of the rolls fails to produce a uniform pressure across the nip formed between the rolls.
One solution to this problem is to form a gradually increasing crown on the roll surface. This crown is in the form of a gradual increase of diameter of the roll toward the center. When a crowned roll is combined with a straight roll or another crowned roll, and the ends of the opposed rolls are brought together, a uniform pressure can be produced across the entire length of the rolls.
A problem associated with crowned rolls is that the rolls only develop a uniform pressure at one selected loading. Thus, as it is often desirable to change the compressive loading depending on the type and thickness of the paper being produced, forming a crowned profile on the rolls is a less than optimal solution.
Another method of crown control involves placing a non-rotating support beam in the center of an outer rotating shell which forms the roll. The support beam supports one long piston, or a number of discrete hydraulic pistons, which engage the inner surface of the roll within the roll in proximity to the nip and force the roll against an opposed roll. Although this method effectively forms a nip of uniform pressure across the length of the roll, the process results in continuous flexure of both rolls, which can lead to fatigue failure in the rolls. Further, with non-ductile materials, such as a granite roll, any flexure in the roll rapidly causes roll deterioration.
This flexure of the rolls can be overcome by employing two self-loading hydraulic crown control systems, one in each opposed roll. Then both rolls have internal, non-rotating support beams and hydraulic cylinders with sliding shoes which engage inside surfaces of the rolls opposite the nip and are free to move in the radial direction in line with the nip plane. The pistons support each crown of the rolls and produce a uniform pressure without deflection of either roll.
In practice, however, the equilibrium is unstable. If either one of the crown support systems has a slightly higher hydraulic pressure or loading, it will tend to cause movement of one roll against the other until one hydraulic cylinder or the other bottoms out. Thus, some form of control system is required for the crown support hydraulic pistons.
German Patent No. 22 30 139 discloses a transversely controlled, hydraulically supported roll in which several supporting pistons are arranged in a row on the cross head along the side of the roller adjacent the nip. Each piston can be independently pressurized with pressure fluid. In this manner, the supporting pistons function to provide predetermined counteracting force that acts against the inner surface of the cylinder to produce a specificline pressure distribution, whereby the cross head, under the influence of the counteracting force, can bend inside the cylinder.
Other patents which include methods for controlling individual cylinders in a crown control support system include German Patent Nos. 22 54 392, 28 49 53, and 30 26 865.
In addition to being costly, crown control via valves controlling pressure to individual cylinders involves controlling the pressures in lines to the support cylinders by means of control valve arrangements. Therefore, the safe operation of these apparatus can only be as effective as the operability of the valve arrangement they employ.
U.S. Pat. No. 4,941,250 discloses a method of controlling the position of individual support pistons in a crown control support system by providing a pressure venting port. Said patent discloses positioning the cylinder by a reliable means, via a single conduit which may be formed by a hole cut out in the upper wall portion of the piston recess, or by an oblique drilled duct. However, while such methods provide high reliability, they require predetermination in the fabrication of the crown supported roll system as to where each piston needs to be positioned, presenting some of the same problems presented by a roll with a machined crowned surface, i.e.: the design lacks adaptability and adjustability in positioning of the crown support.
What is needed is a hydraulic crown support mechanism which is adjustable and reliable in operation.